Circuit breaking apparatus and method for pulse operated electronic switch



Aug. 27, 1968 H. SCHIERHOLT CIRCUIT BREAKING APPARATUS AND METHOD FOR PULSE OPERATED ELECTRONIC SWITCH Filed June 1, 1965 1!. m 1 mm 5.6 N V: R mdfl 0 c M 5 f HY B/ United States Patent 6 Claims. cl. 21969) ABSTRACT OF THE DISCLOSURE In combination with a spark-erosion machining power supply having one or more power transistors which are switched on and off to provide machining discharges, a protective circuit for terminating the current flow through the transistors in response to the leakage current through any one of them exceeding a preselected value. The protective circuit includes a control transistor which is efiective when conductive to operate a relay for terminating the current flow through the power transistors. The control transistor is rendered conductive only between discharges when the leakage current through at least one of the power transistors exceeds the preselected value.

This invention relates to a process for the operation of generators with power-transistors for spark-erosion machines and the like.

It is not new to use power-transistors in this type of generator and they have the advantage of having practicallyno inertia and of Switching the required dischargecurrents with good efficiency. However, power-transistors are sensitive to overheating and they occasionally fail because of manufacturing defects. The power transistor, in failing, on rare occasions loses its conductivity which has relatively harmless consequences. In most cases, the transistor loses its controllability and operates like a permanently closed switch with the result that the source of the work-current is continuously connected to the work gap. A permanent arc develops which is only interrupted after a lapse of time by the feed-control. Since the feed-control does not stand under the influence of the failing transistor, a new are is formed after the interruption of the first are by the closing of the distance between electrode and work-piece with the destruction of the work-piece as the final result.

The purpose of the invention is the elimination of this shortcoming of the operation of generators with powertransistors and to set up the process with such sparkerosion machines in such a way that damages due to transistor failure are safely prevented. This problem is solved, according to the invention, by continually making certain that the switch-transistors are operating perfectly. This is accomplished by measuring the current which is flowing in a blocked-condition during the interval between two discharges. In actual operation two types of currents will occur which provide the gauge-value. This can be the flowing of a greatly intensified rest-current. Such an intensified residue or leakage current indicates an unpermissible rise in the transistors temperature which would eventually lead to the destruction of the transistor. The invention provides that when such increased rest-current is detected, the operating-current is interrupted before the transistor can be destroyed. But a current can also be determined which lies within the sizerange of the operating-current. This is an indication that at least one transistor of the whole circuit is destroyed, whether as a result of a temperature increase which persisted for too long a time or a manufacturing defect.

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According to the invention, the part of the work-current which flows over this transistor or, if necessary, the whole work-current is shut oif. This way, at least the work-piece is saved from destruction as are those transistors of the circuit which are not yet damaged.

For carrying out this process it is preferred to use a switch arrangement in which in the final phase of its output the work-current source is connected to the workgap via one or more switch-transistors, in parallel, with emitter and collector resistors. According to the invention, the voltage drops at the emitter-resistors can be used to shut off the machine. For this purpose these voltage-drops are obtained and, illustratively, fed to a gaugerelay via a diode-grid or a gate-connection which only permits passage under the influence of the control-impulses of the work-current during the interval between two discharges. The gauge-relay shuts off the work-current if at least one of the obtained voltages exceeds a pre-determined value.

The drawing illustrates a preferred circuit-arrangement for carrying out the process.

In the terminal phase of the output of this arrangement the Work-current-source is connected to the workgap via one or more parallel switched transistors with emitter and collector resistors. The current-source 1 is connected to the work-gap 5 in, series with the emitterresistor 2, the switch-transistor 3 and the collector-resistor 4. The drawing shows only one power-transistor 3 with the appropriate resistors 2 and 4. Naturally, as known and practiced, there could be two or more such transistors which would also have to be provided with the required resistors 2 and 4. The base of transistor 3 is connected to a timer which is not shown which, for instance, generates rectangular impulses as indicated at 6. The current which flows through transistor 3 and spark-gap 5 has the same impulse-form sequence as that indicated at 6 as long as transistor 3 is functioning. A relatively high voltage occurs across emitter-resistor 2 during the work-impulse. During the interval between two discharges, e.g. during the interval between the work-impulses, that voltage drops to a small value which is in accordance with the generally acceptable rest-current of the transistor. If one of the transistors 3 were destroyed or showed too high a temperature, it results in an increased voltage across resistor 2 during the interval between two discharges. The voltage-drop at the emitter is obtained and fed to the base of transistor 9 via the diode 7 and resistor 8. If two or more power transistors are used, each of the respective emitterresistors 2 should be fed to the base of the transistor 9 via a separate diode. Such diodes are indicated by 7' and 7". The diode-grid or or gate formed in this manner provides that each individual emitter-resistor 2 acts selectively on the base of transistor 9. The result is that, if a high voltage-drop occurs at any one emitterresistor 2, the base of the transistor 9 is regulated to enable that transistor to conduct. The collector of transistor 9 receives impulses 6' from the timer (not shown) which are similar to the impulses 6 for the transistor 3 phased so that collector voltage is applied to transistor 9 only during the interval between two discharges. The voltage which appears across emitter-resistor 10 of transistor 9 is then fed to the gauge-relay 11. The gauge-relay is set to control, limit or shut 01f the work-current when the value of the voltage which is fed into it via this gate-circuit exceeds a predetermined value. In the chosen example the voltage is obtained at the emitter-resistor 2. It is also possible to obtain the voltage at the collector-resistor 4 and it is also conceivable to switch a special resistor into the work-circuit and then use it as a gauge-resistor.

The gauge-relay 11, which shuts off the current if necessary could also emit an optical signal to indicate an interference. It will be understood that the gaugerelay may be a latching relay.

What is claimed is:

1. A current cutoff and protection method for power supplies in spark-erosion machines utilizing transistors as power switches and in which said transistors are recurrently switched on and oil to control the supply of power, said method comprising the steps of enabling a device for sensing the leakage current through at least one of the transistors only while that transistor is switched 01f, disabling said device only while that transistor is switched on, and reducing the supply of power through that transistor if the sensed leakage current exceeds a preselected value.

2. A method for protecting power supplies in sparkerosion machines utilizing power transistors as power switches and in which said transistors are recurrently switched on and oil to deliver electrical energy from a source to a work gap in a series of pulses to create a series of discharges across the work gap, said method comprising the steps of enabling only during the interval between two discharges a device for sensing the current flowing through the one of said transistors having the greatest leakage current flowing therethrough, disabling said device only during each of said pulses, and interrupting the delivery of electrical energy to the work gap if the sensed current exceeds a preselected value.

3. In a system for delivering electrical energy to a workpiece and an electrode separated by a gap for removing material from the workpiece in a spark-erosion machining operation and the like, the combination of transistor switch means for controlling the delivery of electrical energy from a source of electrical energy to the workpiece and electrode, means for recurrently switching said switch means on and off for delivering time-spaced pulses of electrical energy to the workpiece and electrode, sensing means for sensing the leakage current through said switch means, means coupled to said sensing mean-s for enabling said sensing means only when said switch means is switched 011 and for disabling said 4 g sensing means only when said switch means is "switched on, and means coupled between said sensing means and said source and eifective when said leakage current exceeds a preselected value for reducing the supply of electrical energy to the workpiece and electrode.

4. In a system for delivering electrical energy to a workpiece and an electrode separated by a gap for. re moving material from the workpiece in aspark-erosion machining operation and the like, the combination of transistor switch means for controlling the delivery of electrical energy from a source of electrical energy to the workpiece and electrode, means for recurrently switching said switch means on and off for-delivering time-spaced pulses of electrical energy to the workpiece and electrode, sensing means effective when enabled to sense the current through said switch means, and means for enabling said sensing means only when said swtiching means is switched 01? and for disabling said sensing means only when said swtiching means is switched on.

5. The combination of claim 4 further including means controlled by said sensing means effective when said sensed current exceeds a preselected value for terminating the supply of electrical energy to the workpiece and electrode.

6. The combination of claim 4 in which said sensing means comprises a control device elfective only upon the concurrent application thereto of a signal indicative that the current through said switch means exceeds a preselected value and an enabling signal, and means for applying said enabling signal to said control device each time said switch means is switched off.

References Cited UNITED STATES PATENTS 2,616,014 10/1952 Ellerby 219l09 2,866,921 12/1958 Matulaitis et a1. 219-69 X 2,871,410 1/1959 Matulaitis 315- 3,217,207 11/1965 Webb 219-69 X RICHARD M. WOOD, Primary Examiner.

R. F. STAUBLY, Assistant Examiner.

Disclaimer and Dedication 3,399,288.-Hans Sahierholt, Iserlohn, Germany. CIRCUIT BREAKING APPARATUS AND METHOD FOR PULSE OPERATED ELECTRONIC SWITCH. Patent dated Aug. 27, 1968. Disclaimer and dedication filed Apr. 21, 1975, by the assignee, Ammcd Industries Incorporated.

Hereby disclaims and dedicates to the Public the remaining term of said patent.

[Ofiicial Gazette August 5, 1.975.] 

